This application relates to optical interferometric modulators for modulating light.
Mach-Zehnder electro-optic modulators modulate the intensity of light based on interference of two optical paths. At least one optical path is designed to have an electro-optic material so that a control voltage can be applied to modify the refractive index of the electro-optic material and hence the total optical path length. An input optical signal is split into two optical signals that are respectively coupled into the two optical paths. The two optical signals undergo different optical path lengths and hence are delayed relative to each other. The two optical signals are then combined and interfere with each other to generate an output optical signal.
The amount of the delay can be adjusted or modulated by the control voltage applied across the electro-optic material. Hence, when the relative delay between the two optical signals is 0, or 2xcfx80, etc., the two signals constructively interfere to produce a maximum intensity output. However, when the relative delay is xcfx80, or 3xcfx80, etc., the two signals destructively interfere to produce a minimum intensity output.
The present disclosure includes techniques for optically monitoring the output optical signals of the above Mach-Zehnder modulators and more generally the output optical signals of optical interferometric modulators that use the optical interference between two optical paths to produce an intensity-modulated output optical signal. Such optical monitoring does not require optically tapping of the output optical signals of such modulators but uses another optical signal that is ordinarily unused in such optical modulators.
A device according to one embodiment includes an input waveguide, an output waveguide, and first and second waveguides formed on a substrate. The first and second waveguides respectively have receiving ends coupled to a port of the input waveguide and output ends coupled to a port of the output waveguide. An optical output coupling mechanism is provided to have one end coupled to the output waveguide and another end coupled to an output optical fiber which receives a guided output optical signal from the output waveguide. The device also includes an optical detector, displaced from the substrate and positioned near the optical output coupling mechanism, to receive an optical monitor signal that is not guided by either the output waveguide or the output optical fiber. In particular, this unguided optical monitor signal is complementary to the guided output signal.
An electro-optic material may be used in either or both of the first and the second waveguides to control the difference in the optical path length for the interference operation. The unguided optical monitor signal may be used to detect a drift in the optical path length difference between the first and second waveguides with respect to a desired value. A feedback control may be used to control the electro-optic material in response to the unguided optical monitor signal to reduce the drift. In addition, the unguided optical monitor signal may be used to monitor other aspects of the device, such the bit error rate in the guided output optical signal.